1. Field of the Invention
The present invention relates to a communication terminal apparatus and radio reception method in a digital radio communication system, and more particularly, to a communication terminal apparatus and radio reception method in a Direct Sequence-Code Division Multiple Access system in which forward transmission directional control is applied using a smart antenna and adaptive array antenna.
2. Description of the Related Art
Conventionally in the case where traffic channel signals and common control channel signals are transmitted with the same transmission method (both with omnidirectional), dedicated physical channel (traffic channel) signals are controlled with a very limit received level (Eb/NO or SIR) providing a required quality by transmit power control to improve the spectral efficiency.
On the contrary, with respect to the common control channel signals, in order to enable all communication terminals in a service area to receive surrounding cell information for a handover, in other words, to enable all the communication terminals in the service area to ensure received qualities higher than a predetermined level, the signals are generally transmitted to the terminals in the service area and around the service area with power higher than that of the traffic channel signals.
Accordingly in the above-mentioned case, in each of the communication terminals, a searcher, which selects a path of the traffic channel signals, in other words, which detects a timing for despreading processing, operates using as references the common control channel signals that have higher power levels than the traffic channel signals without using entire traffic channel signals, to perform the search with high reliability.
In a base station, the common control channel (for example, CCPCH: Common Control Physical CHannel) signals are usually transmitted with omnidirectional (omni or sector), and on the contrary, the traffic channel (DPCH: Dedicated Physical CHannel) signals are sometimes transmitted with directional control using a transmission radiation pattern. In this case, as illustrated in FIG. 1, the base station transmits the traffic channel signals to communication terminal 1 with the omnidirectional. Accordingly communication terminal 1 receives all signals passed through path 1 on which the signals directly arrive at communication terminal 1 from base station 2, and paths 2 and 3 respectively on which the signals are reflected by obstacles 3 and 4, and arrive at communication terminal 1. Therefore a delay profile is as illustrated in FIG. 1B.
On the contrary, in the case where transmission directional control is applied to the traffic channel signals, as illustrated in FIG. 2A, transmission is performed with a radiation pattern indicated with X. In this case, since propagation paths are limited, communication terminal 1 receives signals passed through path 1 on which the signals directly arrive at communication terminal 1 from base station 2, and path on which the signals are reflected by obstacle 4, and arrive at communication terminal 1 from base station 2, and does not receive the signals passed through path 2 on which the signals are reflected by obstacle 3, and arrive at communication terminal 1 from base station 2. Therefore the delay profile is as illustrated in FIG. 2B.
Thus, since the directional controlled traffic channel limits propagation paths thereof, propagation paths are different between the traffic channel and the common control channel, and thereby delay profiles become different. In this case, as can be seen from FIGS. 1B and 2B, path 2 that is not contained in the delay profile of the traffic cannel is present in the delay profile of the common control channel. Therefore there is a problem that the searcher selects a wrong path when operates using the common control channel signals as references as described above.
On the other hand, when the searcher operates using the traffic channel signals as references, since the signals are controlled with the very limit received level providing the required quality by the transmit power control, the reliability of path selection is decreased.